class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
      if(root == NULL) return NULL;
      if(root->val < low){
          TreeNode* node = trimBST(root->right,low,high);
          return node;
      }
      if(root->val > high){
          TreeNode* node = trimBST(root->left,low,high);
          return node;
      }
      root->left = trimBST(root->left,low,high);
      root->right = trimBST(root->right,low,high);
      return root;
    }
};
